Early Spitfire, Herald, Vitesse and GT6s featured a rear suspension design known as ‘Swing Axle’ whereby the driveshaft acted as a lower link in the suspension and the transverse leaf spring (bolted on top of the differential) doubled as the upper link.
Although this design is cheap and easy to manufacture, the handling performance is somewhat flawed. The now infamous ‘wheel tuck’ results in a sudden (and sometimes unpredictable) loss of rear end grip if the driver ‘lifts-off’ or if a rear wheel hits a bump mid-corner.
A particularly bad case of ‘wheel tuck’
Why Does it Happen?
The wheels on a car are located by arms that are effectively ‘pinned’ at each end, so that they pivot about their fixing points and move in a semi-circular arc motion. This means that as the wheel moves up and down (by hitting a bump or as the car rolls in a corner, or both) the wheel itself will also follow an arc.
The shape of this arc is therefore dictated by the length and mounting position of the locating links – be they wishbones, driveshafts or leaf springs.
When designing suspension, one of the most important aspects to consider is how the wheel travel ‘arc’ affects the tyre/road contact patch. Ideally, the tyre should be vertical to maximise the size of the tyre contact patch, though in practice this is pretty much impossible to achieve all of the time.
As the car corners, the forces acting on the tyre cause it to distort and gain positive camber. It is therefore desirable for the suspension to gain a small amount of negative camber to provide some ‘camber compensation’ and get the wheel closer the ideal vertical position.
The biggest flaw with the Swing Axle design is that the upper link of the suspension (the leaf spring) is longer than the lower link (the driveshaft). This means that as the wheel moves up and down, it will always have a tendency to gain large amounts of positive camber.
Coupled with the fact that cornering forces also add positive camber, as described above, it means that the size of the tyre contact patch rapidly decrease as the wheel moves upwards.
The rear tyres are far from the 'ideal' vertical
There is also a second (equally troublesome) problem with the swing axle design, known as ‘jacking’. A full explanation of how/why this happens is beyond the scope of this article, but a brief and simplified description is as follows:
As a car corners, the loads exerted on the wheels/tyres must be transmitted through the suspension links and into the chassis. The way these loads are distributed has a major effect on the suspension itself, which in turn affects its performance.
The position and angle of the suspension links control the distribution of loads. These affects are analysed by looking at a virtual ‘point’ where the centre lines of the suspension links and tyres converge. This point is known as the ‘Roll centre’
The roll centre is the point that car pivots about as it rolls in cornering, and is highly influential in the handling characteristics of a car. Ideally, suspensions should be designed so that they have a low roll centre (somewhere below the centreline of the axle), so that the cornering loads help to force the car down onto the tarmac and thus increase ‘grip’.
The flaw in the Swing Axle design is that it has an unusually high roll centre and so the cornering forces push the car upwards – greatly and suddenly reducing grip by effectively ‘pulling’ the tyre away from the road.
This is why in the pictures shown above, the rear of the car is ‘jacked’ up
The solution developed by Triumph in the 1960s (though interestingly not implemented until the 70s because it was deemed ‘too expensive’ by the bean-counters!) is known as the ‘Swing Spring’.
Rather than having a rigidly-mounted transverse leaf spring, only the lower leaf of the spring is bolted onto the differential. The remaining leaves are mounted in a box with a pivot through it so that the upper leaves are ‘floating’.
This means that the roll centre is no longer fixed in a single position and can move slightly as the wheel moves – projecting cornering loads closer to the ground and thus reducing the ‘jacking’ loads. The tyre is therefore less likely to gain extreme positive camber as with swing axle setup.
However, as the upper leaves of the spring are no longer clamped in place, the roll resistance is reduced significantly (the ability of the spring to resist the car rolling). To compensate for this, a bigger (and therefore stiffer) front anti-roll bar is fitted to reduce body roll.
The result of all of this is that the tendency for sudden oversteer is greatly reduced and the car feels more ‘sure-footed’ when being driven quickly. The biggest improvement will be noticed when hitting bumps midway through a fast corner – instead of a heart stopping sideways ‘hop’ the car will instead feel more balanced.